In extrusion under atmospheric pressure of foamable resin compositions by the use of a slit die such as a T die, it expands in three directions. In this case, the expansion in an extrusion direction of the foamed article is not subject to any limitation, whereas the expansion in the thickness direction and in the width direction is limited by an outlet of the T die, leading to the deformation of the foamed article in a wave form. This phenomenon is called a "corrugation phenomenon". Of these deformations, the deformation in the molding direction due to the limitation in the thickness direction can be prevented by controlling the pulling speed of the foamed article. However, the deformation in the width direction due to the limitation in the width direction cannot be prevented. This phenomenon tends to become more marked as the expansion ratio is increased and also as the clearance of the die slit is decreased.
When the foamed article is cooled by contacting with a roll, for example, if the corrugation occurs, the foamed article comes into contact with the roll unevenly and thus streaks parallel to one another in the molding direction are developed, markedly reducing the appearance of the foamed article.
To overcome the above problems, a method using a circular die has heretofore been known. In accordance with this method, however, a plate-shaped foamed article cannot be obtained unless the resulting circular foamed article is cut and opened. This additional step makes the process complicated and further wrinkles are produced at the time of cutting. Thus the above method is not effective to overcome the problems. Another method proposed to overcome the above problems is to produce a smooth foamed article by the use of a sizing die having various forms, as disclosed in Japanese Laid-Open Patent Application Nos. 25068/73 and 65771/79. This method, however, is also not effective because it has disadvantages in that a special apparatus is needed and thus the method is unsuitable for practical use, and the method is difficult to apply to a foamed article small in thickness.